Induction motor



Patented Aug. 18, 1925.

UNITED STATES 1,550,235 PATENT OFFICE.

EDWIN O. BALLMAN, OF ST. LOUIS, MISSOURI, ASSIGIIO'R TO .BALDOR ELECTRIC COM- PANY, OF ST. LOUIS, MISSOURI, A CORPORATION OF MISSOURI.

INDUCTION MOTOR.

-Application filed January 19, 1920. Serial No. 352,592.

To all whom it may concern:

Be it known that I, EDWIN G. BALL-MAN, a citizen of the United States, and residing at St. Louis, Missouri, have invented the new and useful Improvement in Induction Motors, of which the following is a specification.

This invention relates to electric motors, and more particularly to that type of motors usually designated as induction motors.

In many types of induction motors, pro vision is made for starting by arranging the secondary windings so as to produce high starting torque with comparatively low starting current. As such arrangement is usually not best adapted for running condition, it is necessary to alter the winding connectionsafter the motor has started. Mandy'devices for this purpose have been devise comprising various switching arrangments for changing connections. Whenever changes of circuit connections are made while current is flowing in the circuit, an arc is formed at the point of opening or closing the circuit. Such an are invariably has the effect of burning or injuring the contact surfaces.

One of the objects of this invention, there fore, is to provide means for changing the circuit connections of the motor so as to practically eliminate the burning of the contacts.

In the ordinary induction motor, it is common practice to arrange the secondary winding for a high resistance connection during the starting period and to'change to a low resistance connection by short circuiting that winding at a number of points. Such short circuit is usually made .by connecting together all of a considerable number of points in the windin Under such circumstances, it is very ifficult to control the arcing and consequent burning of contacts of the switching devices on account of the fact that since there are a number of making or breaking points, the arc may take place at any one or more of such points and is beyond control of the switching device.

Another object of this invention, there fore, is to so arrange the circuits that the point of arcing at the switch contacts may be controlled. 1

Some types of single phase motors are subject to variations in torque causing slight fluctuations in speed. If the switching dc fore, is to provide switching contacts adapted to compensate for fluctuations in the motor torque.

. In most of the present types of centrifugal devices used on motors of this kind, the

contacts are pivotally mounted in such a 7 way that the forces of acceleration active during the starting period efi'ect the adjustment of the speed at which the switching device acts.

Another object of this invention, therefore, is to provide a switching device which will be free of interference by the forces of acceleration.

Further objects will appear from the detail description taken in connection with the accompanlying drawing, in which:

Figure 1 is a. general view showing a motor embodying this invention;

Figure 2 is a diagrammatic representation of the secondary winding showing the arrangement of the circuits;

Figure 3 'is a detail view of the switching device; and I Figure 4 is a section on the line 414, Figure 3.

Referring to the accompanying drawing, 1 designates the stator and 2 the rotor of an induction motor. The rotor is mounted on a drive shaft 3 which is mounted in bearings in end plates 4. The stator is provided with a winding 5'.

The rotor is provided with a winding 5. As this invention may be applied to any type of induction motor, whether single or polyphase, the winding 6 may be of any type ordinarily used on such motors. In Figure 2, the winding is shown as connected to a commutator 7 such as would sometimes be used in a single phase induction motor and in some types of polyphase motors. This invention may, however, be applied to windings used without a commutator. The winding 6 is divided intov six, preferably equal, sections by means of taps 8. The taps 8 are shown in Figure 2 as connected to commutator bars. They may, however, just as well be connected directly to the coils.

The taps 8 divide the windin into a number of distinct or clearly defined circuits, for instance, the taps A A form the terminals of one circuit, the taps B B, the terminals of another circuit, and the taps C C, the terminals of a third circuit, the terminals of each circuit being'spaced 180 electrical degrees from each other and these circuits bein displaced from each other by 120'electrica degrees. In a Winding of this kind in which current is drawn both from the commutator and from the taps A A, B B, etc, it is well known that the heating efl'ect is greatest in those parts of the wind in which are nearest the taps. It is also true that the total heating efl'ect for the Whole rotor decreases as the number of taps is increased. This total heating effect is not much greater, however, with six taps shortcircuited than with the entire commutator short circuited, so that the present arrangement gives ood operating characteristics.

In the or inary motor of this kind the short-circuiting connect-ions are made be tween points spaced 120 electrical degrees or less. This means that a volt-age less than the full armature voltage is short-circuited, and consequently, with a given horse-power load, a correspondingly increased current must flow. This increased current must be handled by the short-circuiting switch. In accordance with is invention, however, pairs of points A A, B B, etc., 180 electrical degrees apart are short-circuited separately; that is, each pair independently of other pairs. In this way substantially the maximum rotor voltage is short-circuited and correspondingly less current, is to be handled by each switch. By keeping these circuits separate all possibility of short-circuiting less than substantially the" maximum rotor voltage is eliminated.

In designing any type of switch for opening and closing the circuit while the current is flowing, in order to, completely prevent the burning and pitting of the contact surfaces, it is necessary to provide auxiliary contacts designed to close prior to and to open after the closing or opening respectively of the main contacts, in order that the arc will be confined to the auxiliary contacts which should be designed to relieve the main contacts of all arcing.

In this invention this difiiculty has been overcome by means of the switch shown in Figures 3 and 4, in which 9 represents a supporting member mounted on the shaft 3 and having a flange 10. Mounted on the. flange 10, but insulated from it, by means of insulating strips 11 is a. pair of contacts 12. The contacts 12 are attached to the flange 10 by means of screws 13, suitably insulated from said contacts. The contact members 12 are composed of resilient material and are bent, as shown at 14, so as to 081 the said stirrup. The contact 16- is bored as at 21 to receive a spring 22 whichbears at one end against the bottom of said bore 21 and at the other end against the flange 10. The spring 22 is suitably insulated from the flange 10.

' The end of the shank 20 is provided with a depression adapted to accommodate an insulating block 24:. The block 24 is provided to insulate the contact 16 from a spring arm25 mounted on and electrically connected to one of the contact members 12 and carrying at its other end, a contact piece 26 of tungsten, or other suitable material. A second contact 27 is arranged opposite to the contact 26 for engagement therewith and is mounted on an arm 28 integral with or attached to the other contact member 12.

An insulating bushing 29 surrounding the shaft 3 is provided to insulate the arm 25 from the. shaft.

In operation the spring 22 holds the con tact 16 out of engagement with the contacts 12 when the motor is at stand still. At the same time, the shank 20 through the block 24 holdsthe contact 26 out of engagement with the contact 27 After the motor has started centrifugal force acting 'on the contact 16 tends to move it radially outward. At a predetermined speed, said force overcomes the tension of the spring 22 and moves the contact 16 radially outward. This has the effect first of allowing the arm 25 to move outward and close the contacts 26 and 27, and later to cause the contact 16 to bridge the contacts 12. It is therefore, seen that this switch closes the circuit between the members 12, first at the points 26 and 27 where the arcing will occur and later at the points 15 where the main current will be carried. As the switch opens, the circuit will first be opened at the points 15 and later at the points 26 and 27 so that the are will again be broken at the auxiliary contacts which are made of special material adapted to permit such arcing without serious deterioration. It will be seen furthermore that on account of the resilient mounting of the contact, points 15 they will engage with the contact 16 with a sliding action tending to keep the contact surfaces always clean and in a good condition.

On account of the yielding of the resilient mounting of the contacts 15, they will yield to pressure from the contact 16, and any variation in the motor torque causing fiuctuations in speed, will have the effect merely of changing slightly the position of said contacts without actually opening the circuit or causing chatteringand dancing.

Since the contact 16 is mounted So as to slide radially into contacting engagement and is rigidlyv supported circumferentially, in the bearing 18, as its center of gravity lies within said hearing, it is obvious that the accelerating forces of the rotor will have no effect on the adjustment of the speed at which the switch closes as is usually, the

' case with centrifugally operated elements which are pivotally mounted, since the accelerating forces produce turning moments about the pivots.

Three switches, duplicates of that described, are mounted on the support 9 but displaced 120 from each other. Each of these switchescontrols one of the circuits A A, B B, and C G and is insulated from the others. It will be evident, therefore, that the arcing at all times be'confined to r the auxiliary contacts 26 and 27.

From the foregoing, it is evident that this invention accomplishes its stated objects in the attainment of which it is not to be restricted to the details herein set forth, inasmuch as various minor changes may be made in said details without departing from the spirit or scope of this invention. 7

Having thus described this invention, what is claimed is:

1. An induction motor having a distributed secondary winding, pairs of taps to said winding, the taps of a pair being connected to said winding at points spaced substantially 180 electrical degrees, separate switches adapted to separately short circuit the pairs of taps, and means for-operating sald switches substantially simultaneously.

2. In an electric motor, a rotor having a distributed coil Winding closed upon itself,

. uniformly spaced taps dividing said winding into a pluralit of local circuits, and a centrifugal switcli having pairs of contacts adapted to be short-circuited independently, the contacts of each of said pairs being connected to pairs of said taps spaced 180 electrical degrees.

3. An induction motor having a distributed secondary winding, 9. commutator and brushes for said winding, pairs of taps to said winding, the taps of a pair being con-- nected to said winding at points spaced substantially 180 electrical degrees, and means for short circuiting the pairs of taps substantially simultaneously but separately.

4. An induction motor having a distributed secondary winding, a commutator and brushes for said Winding adapted for starting the motor, pairs of taps to said Winding,

the taps of a pair being connected to said winding at points spaced substantially 180 electrical degrees, separate switches adapted to separately short circuit the pairs of taps, and a centrifugal device adapted to close said switches when the motor reaches its contacts and a movable member adapted to control said auxiliary contacts and to bridge said main contacts.

6. In an electric motor, a rotor having a winding adapted for changing the connections thereof, and means for changing said connections, comprising, a pair of main contacts, a pair of auxiliary contacts and a movable member adapted to control said auxiliary contacts and to bridge said main contacts, said auxiliary contacts being adapted to close prior to and open after the .engagement or disengagement respectively of said bridging contact with said main contact.

7. In an electric motor, a rotor winding adapted 'to be interconnected at different points, and a switch for interconnecting said points comprising a pair of main contacts, a pair of auxiliary contacts, and a movable member adapted to control said auxiliary contacts and to bridge said main contacts.

8. In an electric motor, a rotor having a distributed coil winding adapted for changing the connections thereof, and means for changing said connections, comprising a movable, centrifugally operated contact member, and a relatively fixed contact radially external to said movable contact and adapted to be engaged thereby and to support and yield to the radial centrifugal pressure thereof.

9. In an electric motor, a rotor having a distributed coil winding adapted for changing the connections thereof, and means for changing said connections, comprising, a movable, centrifugally operated contact, and a relatively fixed contact radially external to said movable contact and adapted to be slidingly engaged thereby and to support and yield to the radial centrifugal pressure thereof.

10. In an electric motor, a rotor having a distributed coil winding adapted for changing the connections thereof, means for changing said connections, comprising, a movable, centrifugally operated contact, and a relati-vely fixed contact adapted to yieldingly support the centrifugal pressure of said movable contact, whereby fluctuations in the motor torque are compensated for so as to maintain uniform contact.

11. In an electric motor, having a rotor yielding contact adapted to be engaged by having a distributed coil Winding adapted said movable contact, whereby uncertainties 1 for changing the connections thereof, means in contact due to variations in motor speed for changing said connections, comprising, are compensated for by the yielding engage- 5 a radially movable, centrifugally operated mellt et e Said contacts.

contact laterally supported against circum- In testimony whereof I affix my signature ferential movement by the accelerating this 19th day of December, 1919. forces thereon, and a relatively fixed but EDWIN G. BALLMAN. 

